Caliper measurement by using the traditional wireline multi-finger caliper tools is very difficult under the condition of while drilling. The Logging While Drilling (LWD) caliper tool based on ultrasonic distance measurement possesses the advantages of non-contact, high speed, easy process, and strong anti-jamming. By using three ultrasonic transducers which are equidistantly mounted on the surface, the caliper and the eccentricity are effectively measured. The modular design is utilized for the entire system with the MCU as the core, by drawing lessons from the domestic and international research results of the LWD caliper tool based on ultrasonic distance measurement. In order to design a high precise and eccentricity correcting caliper measurement system, the characteristics of ultrasonic transducer, hardware circuit, and program control are together considered and the collaborative design is used. According to the result of calculation and the parameter characteristics of the real ultrasonic transducers, 1MHz ultrasonic transducers are chosen. Since the variable gain detection circuit and the several gain calibrations are utilized, the first echo signal frontier is captured by the comparator, and the time precision of ultrasonic flight is improved to one period of the ultrasonic transducer oscillation. Finally the measurement precision is effectively improved and the measurement deviation is reached at 1mm.